"It's the best summation of humanity's converging ecological problems and the best roadmap to sovling them, all in one compact package." —David Roberts, Grist on Plan B 4.0: Mobilizing to Save Civilization.

A record heat wave scorched Europe in August 2003, claiming an estimated 35,000 lives. In France alone, 14,802 people died from the searing temperatures—more than 19 times the death toll from the SARS epidemic worldwide. In the worst heat spell in decades, temperatures in France soared to 104 degrees Fahrenheit (40 degrees Celsius) and remained unusually high for two weeks.

This summer's high temperatures also hit other European countries. Germany saw some 7,000 people die from the heat. Spain and Italy each suffered heat-related losses of nearly 4,200 lives. The heat wave claimed at least 1,300 lives in Portugal and up to 1,400 lives in the Netherlands.

In London—which on August 10th recorded its first triple-digit Fahrenheit temperature—an estimated 900 people died from the heat. Heat-related fatalities across the United Kingdom reached 2,045. In Belgium, temperatures higher than any in the Royal Meteorological Society's register dating back to 1833 brought 150 deaths. Since reports are not yet available for all European countries, the total heat death toll for the continent is likely to be substantially larger. (See data.)

August 2003 was the warmest August on record in the northern hemisphere, but according to the projections of the Intergovernmental Panel on Climate Change (IPCC), even more extreme weather events lie ahead. By the end of the century, the world's average temperature is projected to increase by 2.5-10.4 degrees Fahrenheit (1.4-5.8 degrees Celsius). As the mercury climbs, more frequent and more severe heat waves are in store.

Though heat waves rarely are given adequate attention, they claim more lives each year than floods, tornadoes, and hurricanes combined. Heat waves are a silent killer, mostly affecting the elderly, the very young, or the chronically ill.

Under normal circumstances, humans maintain a body temperature around 98.6 degrees Fahrenheit. When subject to extreme heat, the body attempts to maintain this ideal temperature by varying blood circulation and perspiring. When the internal body temperature rises above 104 degrees Fahrenheit, vital organs are at risk. If the body temperature is not brought down, death follows.

The threshold ambient temperature at which more people are at risk for heat-related health problems varies greatly by location. In general, when summer temperatures range 10 degrees Fahrenheit or more above the norm, incidences of heat-related illness increase dramatically. High humidity compounds the effects of high heat by reducing evaporation, rendering perspiration a less-effective cooling mechanism. When excessive heat prevails for more than two consecutive days, the risk of heat sickness and death escalates. Health and social services may be overwhelmed.

Heat waves take the greatest human toll in cities. Urban centers, where the area of heat-absorbing dark roofs and pavement exceeds the area covered by cooling vegetation, are like "heat islands" and can be as much as 10 degrees Fahrenheit warmer than the surrounding countryside. While people in rural areas generally get some relief from the heat when temperatures fall at night, urban areas stay warmer around the clock. Air pollution, which usually is worse in cities than in the countryside, can also exacerbate the health-damaging effects of high temperatures by further stressing the body's respiratory and circulatory systems.

Several of the worst heat waves of the twentieth century occurred in U.S. cities. In 1955, an eight-day run of temperatures over 100 degrees Fahrenheit in Los Angeles left 946 people dead. In 1972, New York City suffered a two-week heat wave that claimed 891 lives. More recently, an extreme heat wave in Chicago in 1995 killed 739 people in a matter of days. Slow political recognition of the threat and an overloaded response system worsened the effects of the weather anomaly.

A lack of public recognition of the danger that high temperatures pose adds to the lethality of heat waves. Heat wave warnings often do not carry the weight of other natural disaster alerts. Except during major outbreaks, heat-related deaths often go unreported, and few governments systematically keep records of them.

Even once a heat wave has passed, politicians are reluctant to acknowledge its toll. Chicago's mayor denied the severity of the city's 1995 heat wave. In Europe, it took over a month for France's government to release heat wave fatality estimates that corroborated estimates from overwhelmed undertakers. Several neighboring governments are still challenging reports from medical examiners.

Even in India, where heat-related fatalities in the thousands during pre-monsoonal high temperatures are no longer uncommon, the National Disaster Management Cell does not classify heat waves as a natural disaster. While accurate data are hard to come by, it appears that India has seen the number of deaths due to heat climb over the years as populations have grown and temperatures have risen. In May 2003, peak temperatures of 113-117 degrees Fahrenheit (45-49 degrees Celsius) claimed over 1,600 lives throughout the country. In the state of Andhra Pradesh alone, some 1,200 people died from the heat. A year earlier, a one-week heat wave with temperatures topping 122 degrees Fahrenheit took over 1,000 lives.

Over the last 25 years the average global temperature rose by 1 degree Fahrenheit, or 0.6 degrees Celsius. The IPCC's projected rise in temperature for this century is a global average, but the temperature is expected to rise more over land, where people live, than over sea. As temperatures continue to climb, the toll of heat waves in individual countries could jump from the thousands to the tens of thousands. The World Meteorological Organization estimates that the number of heat-related fatalities could double in less than 20 years.

Already we are seeing evidence of more frequent heat waves. In India, death tolls from heat that were recorded over an entire summer some 10 years ago are now occurring in just one week. In the United States, a 1998 study of summertime temperatures using data from 1949 to 1995 found that the frequency of extremely hot and humid days and the occurrence of multiple-day heat waves increased significantly during that period. Some of the increase is due to urbanization, a trend that is expected to continue for the foreseeable future.

Although the historical data for heat waves leave much to be desired, we can say with confidence that the August heat wave in Europe has broken all records for heat-induced human fatalities. As awareness of the scale of this tragedy spreads, it is likely to generate pressure to reduce carbon emissions. For many of the millions who suffered through these record heat waves and the relatives of the tens of thousands who died, cutting carbon emissions is becoming a pressing personal issue.

Intergovernmental Panel on Climate Change, Climate Change 2001: The Scientific Basis; Impacts, Adaptation, and Vulnerability; and Mitigation. Contributions of Working Group I, II, and III to the Third Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge and New York: Cambridge University Press). Text and summaries of each report available at http://www.ipcc.ch.